There are numerous methods for manufacturing gear wheels. In the chip-removing soft pre-machining, one distinguishes hobbing, gear shaping, generating planing and (power) skiving. The hobbing and skiving are so-called continuous methods, as shall be explained in the following.
In the chip-removing manufacturing of gear wheels, one distinguishes between the intermitted indexing process or single indexing process and the continuous method, which is partly also called a continuous indexing process or face hobbing.
In the continuous method, for example, a tool comprising cutters is applied in order to cut the flanks of a work piece. The work piece is cut in one clamping continuously, i.e., in an uninterrupted process. The continuous method is based on complex coupled movement sequences, in which the tool and the work piece to be machined perform a continuous indexing movement relative to each other. The indexing movement results from the driving in coordination with respect to the coupledly driving of plural axle drives of a machine.
In the single indexing process, one tooth gap is machined; then, for example, a relative movement of the tool and a so-called indexing movement (indexing rotation), in which the work piece rotates relative to the tool, are carried out, and then the next tooth gap is machined. In this way, a gear wheel is manufactured step by step.
The initially mentioned gear shaping method may be described or represented by a cylinder gear transmission, because the intersection angle (also called intersection angle of axes) between the rotation axis R1 of the shaping tool 1 and the rotation axis R2 of the work piece 2 amounts to zero degrees, as represented schematically in FIG. 1. The two rotation axes R1 and R2 run parallel, if the intersection angle of axes amounts to zero degrees. The work piece 2 and the shaping tool 1 rotate continuously about their rotation axes R2 respectively R1. In addition to the rotational movement, the shaping tool 1 carries out a stroke movement, which is referenced in FIG. 1 by the double arrow shx, and removes chips from the work piece 2 during this stroke movement.
Some time ago a method has been taken up anew, which is called (power) skiving. The basics are aged approximately 100 years. A first patent application with the number DE 243514 on this subject dates back to the year 1912. After the original considerations and investigations of the initial years, skiving was no longer pursued further seriously. Hitherto, complex processes, which were partly empirical, were necessary in order to find a suitable tool geometry for the skiving method.
About in the middle of the nineteen eighties, skiving was taken up anew. It was not until the present-day simulation methods and the modern CNC-controls of the machines, that the principle of skiving could be implemented as a productive, reproducible and robust method. The high durability of present-day tool materials, the enormous high static and dynamical rigidity and the high performance of the synchronous running of the modem machines come in addition.
As shown in FIG. 2, during skiving, an intersection angle of axes Σ between the rotation axis R1 of the skiving tool 10 (also called skiving wheel) and the rotation axis R2 of the work piece 20 is prescribed, which is different from zero. The resulting relative movement between the skiving tool 10 and the work piece 20 is a helical movement, which can be decomposed into a rotational portion (rotatory portion) and an advance portion (translational portion). A generation helical type gear transmission can be considered as a drive technology-specific analogon, wherein the rotational portion corresponds to the rolling and the advance portion corresponds to the gliding of the flanks. The greater the absolute value of the intersection angle of axes Σ, the more the translational movement portion required for the machining of the work piece 20 increases. It causes namely a movement component of the cutting edges of the skiving tool 10 in the direction of the tooth flanks of the work piece 20. Thus, during skiving, the gliding portion of the combing relative movement of the mutually engaging gear wheels of the equivalent helical gear is utilized to carry out the cutting movement. In skiving, only a slow axial feed (also called axial feed) is required and the so-called shaping (pushing) movement, which is typical for the gear shaping, is dispensed with. Thus, also a return stroke movement does not occur in skiving.
The cutting speed in skiving is influenced directly by the rotational speed of the skiving tool 10 with respect to the work piece 20 and the utilized intersection angle of axes Σ between the rotation axes R1 and R2. The intersection angle of axes Σ and thus the gliding portion should be selected such that for a given rotational speed an optimum cutting speed is achieved for the machining of the material.
The movement sequences and further details of an established skiving method can be taken from the schematic representation in FIG. 2 that has already been mentioned. FIG. 2 shows the skiving of an outer toothing on a cylindrical work piece 20. The work piece 20 and the tool 10 (here a cylindrical skiving tool 10) rotate in opposite directions.
There are additional relative movements. An axial feed sax is required in order to be able to machine with the tool 10 the entire toothing width of the work piece 20. If a helical toothing is desired on the work piece 20 (i.e., β2≠0), a differential feed sD is superimposed on the axial feed sax. A radial feed srad may be carried out as a lining movement. The radial feed srad may also be employed in order to influence the convexity of the toothing of the work piece 20.
In skiving, the vector of the cutting speed {right arrow over (v)}c results substantially as the difference of the two velocity vectors {right arrow over (v)}1 and {right arrow over (v)}2 of the rotation axes R1, R2 of the tool 10 and the work piece 20, which [velocity vectors] are tilted with respect to each other by the intersection angle of axes Σ. The symbol {right arrow over (v)}1 is the velocity vector at the periphery of the tool and {right arrow over (v)}2 is the velocity vector at the periphery of the work piece 20. The cutting speed vc of the skiving process may thus be changed by the intersection angle of axes Σ and the rotation speed in the equivalent helical gear. The axial feed sax has only a small influence on the cutting speed vc which can be neglected and is thus not shown in the vector diagram comprising the vectors {right arrow over (v)}1, {right arrow over (v)}2 and {right arrow over (v)}c in FIG. 2.
The skiving of an outer toothing of a work piece 20 using a conical skiving tool 10 is shown in FIG. 3. In FIG. 3 again, the intersection angle of axes Σ, the vector of the cutting speed {right arrow over (v)}c, the velocity vectors {right arrow over (v)}1, at the periphery of the tool 10 and {right arrow over (v)}2 at the periphery of the work piece 20 as well as the cant angle β1 of the tool 10 and the cant angle β2 of the work piece 20 is shown. Here, in contrast to FIG. 2, the cant angle β2 is different from zero. The tooth head of the tool 10 is referenced with the reference sign 4 in FIG. 3. The tooth breast is referenced with the reference sign 5 in FIG. 3. The two rotation axes R1 and R2 do not intersect, but are arranged skew (skew-whiff) with respect to each other. For a conical skiving tool 10, the calculation point AP is hitherto usually chosen on the joint plumb of the two rotation axes R1 and R2, because a tilting of the skiving tool 10 for providing of end relieve angles is not necessary. The calculation point AP coincides with the so-called contact point. The rolling circles of the equivalent helical generation gear contact each other in this calculation point AP.
In order to make the productivity of the skiving—for example when applying modern cutting materials such as hard metals for dry machining—as large as possible, the gliding portion of the relative movement between the skiving tool and the work piece must produce sufficiently high cutting speeds. In skiving, the cutting speed vc is influenced directly by the rotation speed of the equivalent helical gear, by the effective work piece with respect to tool radii and by the intersection angle of axes Σ of the rotation axes R1 and R2. The possible rotation speed is limited here by the permitted rotational frequency of the machining apparatus (skiving machine) used. The size of the work piece is fixedly predetermined. The possible size of the tool is limited by the work space of the machining apparatus (skiving machine) employed and for inner toothings also by the inner space of this proper toothing. Therefore, sufficiently high cutting speeds can often be generated only by corresponding large intersection angles of axes Σ.
The intersection angle of axes Σ, however, cannot be predetermined arbitrarily in practice, because beside the purely vectorial consideration of the different movements, which are superimposed, there are a number of other aspects, which must be taken into account compulsorily. These additional aspects, which must be incorporated in the considerations, are described in the following paragraphs.
In skiving, a tool 10 comes to application, which comprises at least one geometrically determined cutting edge. The cutting edge/cutting edges are not shown in FIG. 2 and FIG. 3. The shape and arrangement of the cutting edges belong to those aspects, which must be taken into account for a concrete layout in practice.
In addition, the tool itself has a great importance in skiving. In the example shown in FIG. 2, the skiving tool 10 has the shape of a spur-toothed spur wheel. The outer contour of the base body in FIG. 2 is cylindrical. However, it can also be tapered (also called conical), as shown in FIG. 3. Because the tooth or the teeth of the skiving tool 10 come in engagement along the entire length of the cutting edge, each tooth of the tool 10 requires a sufficient end relieve angle at the cutting edge.
When starting from a spur-toothed or a helically toothed conical skiving tool 10 as shown in the FIGS. 4A and 4B, then one recognizes that such a skiving tool 10 has so-called constructive end relief angles (respectively rake angles) due to the conical basic shape of the skiving tool 10, i.e., the end relieve angle at the head and on the flanks of the conical skiving tool 10 are predetermined due to the geometry of the skiving tool 10. However, the profile of the cutting edge of a conical skiving tool 10, must obey certain conditions, in order to enable reshaping at all. In the FIGS. 4A and 4B, a conical skiving tool 10 is shown when generating an outer toothing on a work piece 20. The so-called constructional rake angle αKo at the cutter head of the conical skiving tool 10 is visible in FIG. 4B. The intersection point of axes AK and the contact point BP of the rolling circles of the skiving tool 10 and the work piece 20 coincide in FIG. 4A and lie on the joint plumb GL (not shown in FIGS. 4A and 4B) of the rotation axes R1 and R2.
In FIG. 5, a further representation of a spur-toothed or helical-toothed conical skiving tool 10 and a cylinder-shaped work piece 20 is shown, wherein the view of FIG. 5 has been chosen such that both rotation axes R1 and R2 run parallel, although the two axes R1 and R2 are skew with respect to each other. The joint plumb GL of the two axes R1 and R2 can be seen in FIG. 5. The contact point BP lies on the joint plumb GL as shown in FIG. 5.
In FIGS. 6A and 6B, a configuration of a cylindrical skiving tool 10 and an outer-toothed cylindrical work piece 20 is shown. The skiving tool 10 is not only skewed with respect to the rotation axis R2 of the work piece 20 (as can be recognized in FIG. 6A on the basis of the intersection angle of axes Σ), but is positioned with respect to the work piece 20 such that it is tilted away from it by a small angle αKi (as is seen in FIG. 6B). By tilting the skiving tool 10 away, an effective rake angle can thus be generated, which is shown in FIG. 6B for the head cutting edge as αKi. Effective rake angles are also generated at the lateral cutting edges of the tool by the tilting away. However, these turn out to be smaller than at the head cutting edge. As a general rule, these rake angles are only half as large.
When starting from a spur-toothed or a helically toothed cylindrical skiving tool 10, as shown in the FIGS. 6A and 6B, one recognizes that such a skiving tool 10 does not have so-called constructional rake angles, neither at the head nor at the flanks. If such a cylindrical skiving tool 10 was clamped in the conventional manner, there would be no rake angles. A kinematic rake angle can be generated by the tilting away of the skiving tool 10 as already described. In practice, the tilting away of the skiving tool 10 is achieved by an eccentric clamping of the skiving tool 10 in the machine, in order to thus cause an offset of the cutting face from the intersection point of axes AK. The contact point BP of the rolling circles of the skiving tool 10 and the work piece 20 no longer lies on the joint plumb of the rotation axes R1 and R2 due to the tilting away of the skiving tool 10. The resulting offset is also called cutting face offset e and is recognizable in FIG. 6A. The further the skiving tool 10 is tilted away, the larger the effective rake angles become. The rake angles required for skiving lie in the range between 3 degrees and 5 degrees. In order to prescribe these rake angles, a tilting away of cylindrical skiving tools 10 of up to 10 degrees is required and usual in practice.
In the FIGS. 7A and 7B, further illustrations of a spur-toothed and a helically toothed skiving tool 10 and a cylindrical work piece 20 are shown, whereby the view of FIG. 7A has been chosen such that both rotation axes R1 and R2 run parallel to each other, although the two axes R1 and R2 are skew with respect to each other. In FIG. 7A, the joint plumb GL of the two axes R1 and R2 can be recognized. In the FIGS. 7A and 7B, the contact point BP is located above the joint plumb GL. In FIG. 7B, a so-called contact view (also called side projection of contact plane) is shown, in which the contact point BP is visible. In the representation of FIG. 7A, the contact point BP lies hidden behind the work piece 20.
Beside the kinematic aspects and the conditions, which result from the prescription of the desired clearance angles, also the properties and condition of the work piece 20 plays a role that is not unimportant. Again and again, there are work pieces, in which a section having a greater diameter than the diameter of the root circle joins the toothing or the periodical structure and which therefore allow only a small overrun in the manufacture of a toothing or another periodical structure.
In the FIGS. 8A and 8B, an example of a work piece 20 is shown, which comprises a first cylindrical section 21 and a second cylindrical section 22, wherein an outer toothing is to be manufactured on the first cylindrical section 21 by means of skiving by application of a conical skiving tool 10. The work piece 20 may, for example, concern a shaft comprising sections having different diameters. An effective intersection angle of axes Σeff of at least 10 degrees is required for achieving a sufficient cutting speed vc. It can be recognized both in FIG. 8A and also in FIG. 8B, that the conical skiving tool 10 with an effective intersection angle of axes Σeff of at least 10 degrees would collide with the second cylindrical section 22 of the work piece 20 for a common clamping. The collision section is schematically indicated by an oval KB in FIG. 8B. For a cylindrical skiving tool 10, as shown, e.g., in FIG. 7A, a collision would also result, whereby the situation there is even worse due to the additional tilt.